A phase change type optical recording medium utilizing the phase transfer between crystalline and amorphous, is known as one of optical recording media capable of recording, retrieving or erasing information by irradiation with light such as a laser beam. The structure of such a phase change type optical recording medium usually takes a multilayer structure as shown in FIG. 1. Namely, on a substrate 1 having concavo-convex, located on the incident side of a laser beam, a phase change recording layer 3 (in this specification, the phase change recording layer may sometimes be referred to simply as a recording layer) made of a phase change material which is usually sandwiched by protective layers 2 and 4 made of a dielectric material such as a mixture of ZnS and SiO2, is formed. Further, on the protective layer 4, a reflective layer 5 composed mainly of Au, Al, Ag or the like as the main component, which serves as a heat diffusion layer, is formed. On the reflective layer 5, an overcoat layer 6 made of an ultraviolet curable resin or the like, is formed. As a typical material system for the recording layer 3, a Ge2Sb2Te5 system having a composition close to an intermetallic compound, or a system having a composition close to Sb0.7Te0.3, eutectic point of Sb—Sb2Te3, as the main component, may be mentioned.
The recording principle of such a phase change-optical recording medium is as follows. After the film formation, the above recording layer is amorphous, and accordingly, the reflectance of the phase change type optical recording medium is low. Therefore, after formation of the recording layer, the phase change type optical recording medium is irradiated with a laser beam to heat the recording layer to carry out the step of crystallizing the recording layer over the entire surface of the substrate to let the entire surface of the phase change type optical recording medium have a high reflectance (in this specification, this step may be referred to as initial crystallization). Such an initially crystallized phase change type optical recording medium is locally irradiated with a laser beam, so that the recording layer is melted and quenched and thereby changed to an amorphous state. Along with this phase change, the optical nature of the recording layer changes, whereby information will be recorded. Retrieving of information is carried out by irradiating a laser beam weaker than during the recording, and detecting the difference in the optical nature between the crystalline state and the amorphous state as a difference in the reflectance. Rewriting of information is carried out by directly rewriting a record mark present in the recording layer to a new record mark by irradiating the optical recording medium with a laser beam having a writing peak power superposed on an erasing power with a low energy to cause crystallization.
With a rewritable phase change type optical recording medium, as rewriting is repeated many times, there may be a case where mutual diffusion of constituting atoms or a chemical reaction will take place between the recording layer 3 and protective layers 2 and 4 in contact therewith. Such mutual diffusion of constituting atoms or a chemical reaction will deteriorate the quality of a record mark in such a form as a decrease in the signal amplitude of a record mark or an increase in the jitter value, whereby the number of repetitive rewriting times of the optical recording medium will be reduced.
In order to solve such a problem, a method has been proposed to provide a single layer of a diffusion preventing layer made of e.g. GeN between the recording layer and the protective layer (WO 97/34298). Such a diffusion preventing layer of e.g. GeN has a function to prevent mutual diffusion of constituting atoms between the protective layer and the recording layer, and it is thereby intended to improve the number of repetitive rewriting times for rewriting information.